The present invention generally relates to a charged particle beam apparatus and in particular concerns alignment of the center axis of a charged particle beam with that of an aperture in the charged particle beam apparatus.
In the charged particle beam apparatus, an electron beam is commonly used. As a typical application of such charged particle beam apparatus, there may be mentioned an electron beam pattern writing apparatus for forming on a semiconductor wafer a pattern of circuits for fabrication of a semiconductor integrated circuit. The pattern can be formed either by using a mask or by scanning with the electron beam.
The electron beam pattern writing apparatus for forming the circuit patterns by scanning with the electron beam is usually provided with at least two aperture members each having a rectangular aperture formed therein. These two aperture members are adapted to be moved relative to each other in the direction perpendicular to the axis of the electron beam, whereby a minute rectangular aperture image of predetermined size and shape is projected onto a target which may be a semiconductor wafer to be worked. By scanning two-dimensionally the target with the rectangular aperture image (e.g. electron beam of a rectangular cross-section), desired patterns can be formed on the target as if they were stamped.
The electron beam irradiating apparatus of the variably shaped beam type described above which includes plural stages of electron lenses, apertures and deflecting units as well as a number of focusing elements and thus necessarily requires an elongated path for the focusing of the electron beam suffers a serious drawback that the rectangular image focused on the target is likely to be distorted or disturbed due to irregular electric fields produced by deposition of pollutant on the aperture members, the pollutant containing as main components carbon particles produced upon collision of the electrons with floating gas molecules, and/or due to slight mechanical distortions and deformations of the mechanical elements under the influence of variations in temperature or other environmental factors. Heretofore, this kind of problem has been disposed of by manually adjusting appropriately the deflecting direction as well as the intensity of deflection, which however requires a lot of time and provides a major cause for degrading the efficiency of the pattern writing operation. In particular, when the apparatus has been disassembled for the cleaning purpose, difficulty is encountered in attaining or re-establishing the optimally adjusted or aligned state within a reasonably short time even for the skilled person.